Beside first order defocus and astigmatism of the eye a number of other vision defects could be present. For example aberrations of different orders occur when a wavefront passes a refracting surface. The wavefront itself becomes aspheric when it passes an optical surface that has imperfections and vision defects occur when an aspheric wavefront falls on the retina. Both the cornea and the lens in the capsular bag contribute thus to these types of vision defects if they deviate from being perfect or perfectly compensating optical elements. The term aspheric will in this text include both asphericity and asymmetry. An aspheric surface could be either a rotationally symmetric or a rotationally asymmetric surface and/or an irregular surface, i.e all surfaces not being spherical.
It is presently discussed that the visual quality of eyes having an implanted intraocular lens (IOL) is comparable with normal eyes in a population of the same age. Consequently, a 70-year-old cataract patient can only expect to obtain the visual quality of a non-cataracteous person of the same age after surgical implantation of an intraocular lens, although such lenses objectively have been regarded as optically superior to the natural crystalline lens. This result can be explained by the fact that present IOLs are not adapted to compensate for age-related defects of the optical system of the human eye. Age-related defects of the eye have also recently been investigated and it is found that contrast sensitivity significantly declines in subjects older than 50 years. These results seem to comply with the above-mentioned discussion, since the contrast sensitivity measurements indicate that individuals having undergone cataract surgery with lens implantation will not obtain a better contrast sensitivity than persons of an average age of about 60 to 70 years.
Even if intraocular lenses aimed to substitute the defective cataract lens and other ophthalmic lenses, such as conventional contact lenses or intraocular correction lenses, have been developed with excellent optical quality, it is obvious that they fail to correct for a number of aberration phenomena of the eye including age-related aberration defects.
U.S. Pat. No. 5,777,719 (Williams et al.) discloses a method and an apparatus for accurately measuring higher aberrations of the eye as an optical system with wavefront analysis. By using a Hartmann-Shack wavefront sensor, it is possible to measure higher order aberrations of the eye and using such data to find compensation for these aberrations and thereby obtain sufficient information for the design of an optical lens which can provide a highly improved optical correction. The Hartmann-Shack sensor provides means for obtaining light reflected from the retina of the eye of a subject. The wavefront in the plane of the pupil is recreated in the plane of the lenslet array of the Hartmann-Shack sensor. Each lenslet in the array is used to form an aerial image of the retinal point source on a CCD camera located at the focal plane of the array. The wave aberration of the eye, in the form of a point source produced on the retina by a laser beam, displaces each spot by an amount proportional to the local slope of the wavefront at each of the lenslets. The output from the CCD camera is sent to a computer, which then performs calculations to fit slope data to the first derivatives of 65 Zernike polynomials. From these calculations, coefficients for weighting the Zernike polynomials are obtained. The sum of the weighted Zemike polynomials represents a reconstructed wavefront distorted by the aberrations of the eye as an optical system. The individual Zernike polynomial terms will then represent different modes of aberration.
U.S. Pat. No. 5,050,981 (Roffman) discloses another method for designing a lens by calculating modulation transfer functions from tracing a large number of rays through the lens-eye system and evaluating the distribution density of the rays in the image position. This is repeatedly performed by varying at least one lens surface until a lens is found which results in a sharp focus and a minimum of image aberrations.
The methods referred to above for designing are suitable for the design of contact lenses or other correction lenses for the phakic eye which can be perfected to compensate for the aberration of the whole eye system. However, to provide improved intraocular lenses adapted to be placed between the cornea and the capsular bag, in the anterior chamber or in the posterior chamber, it would be necessary to consider the aberrations of the individual parts of the eye.
There has recently been a focus on studying the aberrations of the eye, including a number of studies of the development of these aberrations as a function of age. In one particular study, the development of the components of the eye were examined separately, leading to the conclusion that the optical aberrations of the individual components of younger eyes cancel each other out, see Optical Letters, 1998, Vol. 23(21), pp. 1713–1715. Also the article of S. Patel et al in Refractive & Corneal Surgery, 1993, Vol. 9, pages 173–181 discloses the asphericity of posterior corneal surfaces. It is suggested that the corneal data can be used together with other ocular parameters to predict the power and the asphericity of an intraocular lens with the purpose of maximizing the optical performances of the future pseudophakic eye. Furthermore, it was also recently observed by Antonio Guirao and Pablo Artal in IOVS, 1999, Vol. 40(4), S535 that the shape of the cornea changes with age and becomes more spherical. These studies indicate that cornea in the subjects provides a positive spherical aberration which increases with the age. In Vision Research, 1998, 38(2), pp. 209–229, A Glasser et al. investigated the spherical aberration of natural crystalline lenses from eyes obtained from an eye bank after that the corneas had been removed. According to the laser scanner optical method used herein it was found that the spherical aberration from an older lens (66 years) shows uncorrected (positive) spherical aberration, whereas a 10-year-old lens shows over-corrected (negative) spherical aberration.
In view of the foregoing, it is apparent that there is a need for ophthalmic lenses that are better adapted to compensate the aberrations caused by the individual surfaces of eye, such as the corneal surfaces and the surfaces of the lens in the capsular bag, and capable of better correcting aberrations other than defocus and astigmatism, as is provided with conventional ophthalmic lenses.